You've logged more than a few hours practicing. The desk has a little bit of wear where you set the sole of your practice chanter. Your gracenotes are becoming tidy. Your embellishments are becoming consistent and crisp. Your natural inclination is to reach over to the metronome and crank the dial up.
You still can reap the benefits of practicing slowly.
But I want to play fast! Everybody does. Before you step on the gas, though, remember Ithzak Perlman's admonishment regarding practice: "it has to be slow, do not practice impatiently!"
When you were working on getting your gracenotes and embellishments up to speed, you were able to focus on making them neat and tidy by practicing slowly. The lower tempo gave you the time that you needed to properly execute the movements. Now that your gracenotes and embellishments are crisp, they do not take as much time as you play through each bar of music. If you continue to practice slowly, it will give you "time" to work on perfecting scale navigation, basic rhythm, and ALAP/ASAP.
Scale Navigation. Are your movements from note to note clean? Are your fingers moving consistently and, when necessary, simultaneously, so that you do not have crossing noises?
Crossing Noises. They are pernicious and can creep in as you step on the gas. Crossing noises don't just creep in of their own accord, though. They happen for a reason. They happen because of improper technique. For example, if you are playing D and attempting to land your newly minted, crisp G grace note on E, are you lifting the high G, E, and low A fingers at the same time. As you execute that crisp G grace note on E (and precisely on the beat), are the D, C, and B fingers coming down on the chanter at the exact same time as the crisp G grace note? When you practice slowly, you can focus on and reinforce these movements so that when you do bring the tune up to speed, crossing noises will not creep in.
Tricky movements. We all have an embellishment, or several, that gives us fits. You may, for example, consistently miss an E doubling coming down from F. This may happen if you had that tendency before you developed your tidy doublings. If you practice slowly, you can still execute those neat and tidy doubling but focus on making the proper movements to execute a doubling on E from F.
Basic rhythm. Are you being precise with the rhythm? Is the first step of each doubling landing on the beat? When you practice slowly, you can ensure that you are keeping a steady tempo and those neat and tidy embellishments will sound great when they land on the beat.
ALAP/ASAP: With your mad new skills in tow, this is where you can really make your tunes shine by practicing slowly. This is especially true for D throws that are executed in the Dojo U style (low G falls on the beat). Consider the second bar of the strathspey "Highland Harry":
With a crisp throw, we can really bring out the implied dynamic in the first beat by using ALAP/ASAP. If we stretch out the G, playing it as an ALAP, follow it with the E as an ASAP, and the play a clean, crisp, and even throw to D, we really bring the emphasis on to the first beat:
We can really work the ALAP/ASAP thoroughly by using metronome magnification and practicing slowly. If we set the metronome to 100 bpm, with four clicks per beat, we can easily determine the length of the ASAP (after the fourth click but before the click for the second beat) and then deduce the length of the ALAP from there (clicks one, two, three, and part of four). It is interesting to note that when we can execute the ALAP G to ASAP A to a really crisp D throw, we really begin to sound like a piper, even at the slower tempos (it's pretty cool when you finally get it).
We do eventually want to pick up the tempo. With neat and tidy doublings, you will find that it is easier to step on the gas. However, you always want to keep a slow practice routine in your tool kit. When you practice slowly, you can continue to refine you ability to navigate the scale, you can maintain and develop your sense of rhythm, and you can really work your ALAP/ASAPs.
Catlodge - Parts 1 and 2 [Vintage]
Bruce Gandy - Slow Airs [Vintage]
Revamping Your D-Throw (with Andrew Douglas)